Production of concentrated carbonate suspensions

ABSTRACT

A method of producing a rheologically stable concentrated aqueous suspension of a particulate alkaline earth metal carbonate which method comprises the following steps: (a) preparing a dilute aqueous suspension of the carbonate having a solids content of not more than 40% by weight; (b) dewatering the dilute aqueous suspension to form a carbonate suspension having a solids content in the range of from 45% to 65% by weight; (c) optionally mixing with the dewatered suspension formed in step (b) a dispersing agent for the carbonate to form a fluid suspension; (d) further dewatering the fluid suspension formed in step (b) by thermal evaporation under reduced pressure to raise the solids content of the suspension by a further differential amount of at least 5% by weight; and (e) after at least some of the dewatering in step (c) treating the suspension by a mechanical working process in which at least 1 kW.hr per tonne of carbonate on a dry weight basis is dissipated to refluidise the suspension.

This application is a 371 of PCT/GB 99/04249, filed Dec. 23, 1999, whichclaims priority based on United Kingdom 9828514.1 filed Dec. 23, 1998and United Kingdom 9910038.0 filed Apr. 30, 1999.

The present invention relates to the production of concentratedcarbonate suspensions, especially calcium carbonate suspensions.

This invention concerns a cost effective process for increasing thesolids concentration of a dilute aqueous suspension of a particulatecarbonate especially calcium carbonate such as precipitated calciumcarbonate, to form a concentrated suspension which is, on the one hand,sufficiently fluid to enable it to be pumped and delivered through pipesor hoses, but, on the other hand, sufficiently viscous to prevent theformation of a sediment of the coarser particles present.

An important factor to be considered if a suspension of particulatecarbonate is to be transported in the form of a concentrated aqueoussuspension, is that the freight charges will generally be based upon thetotal weight of the suspension, and it is therefore desirable tominimise the weight of water, and maximise the weight of particulatecarbonate in the suspension. Generally, it is found that the solidsconcentration of the concentrated suspension should be in the range offrom about 65% by weight to about 80% by weight of particulate carbonateon a dry weight basis. The calcium carbonate may be prepared in the formof a dilute suspension For example, calcium carbonate prepared by asynthetic route is generally precipitated under conditions such that theproduct of the precipitation step is a dilute aqueous suspension havinga solids content in the range of from about 5% to about 40% by weight.Most frequently, the solids concentration of this suspension will be inthe range of from about 15% to about 25% by weight.

A process which has been proposed for increasing the concentration of adilute aqueous suspension to form a concentrated suspension suitable fortransport and storage comprises dewatering the dilute suspension in atube pressure filter of the type described in GB-A-1240465. This type ofpressure filter can be operated at a pressure of up to about 100 bar ormore, and can dewater a dilute suspension of precipitated calciumcarbonate to produce a cake containing from about 70% to about 74% byweight of calcium carbonate. The cake thus formed must then be mixedwith an aqueous solution of a dispersing agent for the calcium carbonateto form a fluid, Theologically stable suspension. A process of thisgeneral type is described in EP-A-0768344. Unfortunately this processsuffers from the disadvantages that, firstly, the tube pressure filterhas been found to be expensive in both capital and maintenance costs,and, secondly, that a considerable amount of energy is required to mixthe cake which is formed by the tube pressure filter with a solution ofa dispersing agent in order to form a fluid and rheolgically stablesuspension.

Other methods have been proposed, e.g. as in U.S. Pat. No. 4242318, toconcentrate dilute carbonate dilute suspensions but these do not attainthe highest solids concentration possible.

The present invention concerns a method for economically producing adewatered aqueous suspension of a particulate carbonate which suspensionis fluid and Theologically stable and contains a high solids Contentwhich is at least 65%, and is preferably from 70% by weight to about80%.

According to the present invention there is provided a method ofproducing a rheologically stable concentrated aqueous suspension of aparticulate alkaline earth metal carbonate which method comprises thefollowing steps:

(a) preparing a dilute aqueous suspension of the carbonate having asolids content of not more than 40% by weight;

(b) dewatering the dilute aqueous suspension to form a carbonatesuspension having a solids content in the range of from about 45% toabout 65% by weight;

(c) optionally mixing with the dewatered suspension formed in step (b) adispersing agent for the carbonate to form a fluid suspension;

(d) further dewatering the fluid suspension formed in step (b) bythermal evaporation under reduced pressure to raise the solids contentof the suspension by a further differential amount of at least 5% byweight; and

(e) after at least some of the dewatering in step

(c) treating the suspension by a mechanical working process in which atleast 1 kW.hr per tonne of carbonate on a dry weight basis is dissipatedto re-fluidise the suspension.

Dispersing agent is preferably added before step (d) in the method ofthe invention, but, if not added at this stage, is preferably addedprior to or during working in step (e). If dispersing agent is addedprior to evaporative dewatering in step (d), further dispersing agent inone or more doses may be added to the suspension after at least some ofthe evaporative dewatering in step (d).

The suspension may following the said working step (e) and followingfurther optional dispersing agent addition be dewatered to give afurther differential increase in the solids content of the suspension bythermal evaporation under reduced pressure. The further differentialincrease in solids content after working after some evaporativedewatering may be at least 5% by weight.

The suspension at its maximum concentration following the evaporativedewatering step(s) may be treated by further working and/or dispersingagent addition prior to storage or use.

The suspension obtained as a product of the method of the invention maybe a fluid and Theologically stable aqueous suspension of the carbonatehaving a solids concentration of at least 65% by weight, e.g. at least68% by weight, in many cases from 70% to 80% by weight.

Preferably, the or each addition of the dispersing agent is accompaniedby stirring of the suspension either whilst the dispersing agent isadded or after the addition of the dispersing agent. The said workingapplied may be provided by the said stirring, e.g., by applying thestirring in a blunger, or may be applied separately from and additionalto the said stirring.

The working may be applied in at least two steps, namely after some ofthe said evaporative dewatering followed by further evaporativedewatering and then again later after the further evaporativedewatering. In addition, working may optionally be applied prior to theevaporative dewatering. The working in each in each case may be appliedby a mechanical device in one of the ways described later.

The particulate carbonate prepared in step (a) may comprise calciumcarbonate although it will be apparent to those skilled in the art thatthe method of the invention may also be applied to other carbonates suchas magnesium carbonate and barium carbonate.

The carbonate in step (a) may be prepared from a natural source, e.g.chalk, marble or limestone, by known grinding procedures. Alternatively,the carbonate may be synthesised. The invention is particularly suitableto treat calcium carbonate which has been prepared by carbonation oflime in a dilute aqueous medium, e.g. by use of carbon dioxide. Thecarbonate produced in this manner is known as precipitated calciumcarbonate.

Where the carbonate prepared in step (a) is a precipitated calciumcarbonate, it may be in the calcite, aragonite or vaterite form or amixture of two or more of these forms or different shapes thereof. Theconditions required to produced these various crystal forms and variousshapes thereof are well known to those skilled in the art. The vateriteform is thermodynamically unstable at normal temperatures. The aragoniteform is metastable under normal ambient condition, but converts tocalcite at high temperatures. The calcite form is the most stable, andcan exist in several different crystal shapes of which the rhombohedraland scalenohedral shapes are probably the most important.

In step (b) of the method of the invention, the dewatering may beeffected by one or more known mechanical devices, e.g. by means of aplate filter press, a vacuum filter, a membrane cross-flow filter or acentrifuge or two or more of these devices. A plate filter press is mostpreferred. Where a plate filter press is used, it is preferably operatedat a pressure in the range of from about 3 bar (300 kPa; 43.5 psig) toabout 20 bar (2000 kPa; 290 psig). The plate filter press mayadvantageously be of the type in which each chamber in which a cake isformed is provided with a flexible diaphragm which is biased against thecake formed on the filter medium by hydraulic or pneumatic pressure inorder to expel more water through the pores of the filter medium It willbe apparent to those skilled in the art that a known flocculant may beemployed to facilitate operation of the dewatering in step (b). Thesolids content of the suspension prior to step (b) may be less than 30%by weight, in some cases not more than 25% by weight, and the solidscontent after completion of step (b) and prior to step (c) may begreater than 50% by weight, e.g. from 50% to 60% by weight.

Where dispersing agent is added in at least two steps or doses, thedispersing agent employed may be the same or different. In each case,the dispersing agent is preferably introduced in the form of aconcentrated aqueous solution. In each case, it may be added in one ormore addition doses at one or more addition sites. The dispersing agentin each case may be selected from the dispersing agents known in the artfor the dispersion of particulate carbonate in an aqueous medium. Thedispersing agent may, for example, comprise an inorganic agent and/or anorganic agent. As inorganic agent, a condensed polyphosphate may beemployed. As organic agent an organic polyelctrolyte may be employed.The polyelectrolyte may comprise a polycarboxylate which may be ahomopolymer or a copolymer which contains a monomer unit comprising avinyl or olefinic group which is substituted with at least onecarboxylic acid group, or a water soluble salt thereof. Examples ofsuitable monomer units are acrylic acid, methacrylic acid, itaconicacid, chronic acid, fumaric acid, maleic acid, maleic anhydride,isocrotonic acid, undecylenic acid, angelic acid and hydroxyacrylicacid. The number average molecular weight of the polycarboxylatedispersing agent should not be greater than 20,000, and preferably inthe range of from 700 to 10,000, as measured by the method of gelpermeation chromatography using a low angle.laser light scatteringdetector. A particularly preferred dispersing agent, on account of itsrelatively low cost and ready availability, is partially or fullyneutralized sodium polyacrylate. The total effective (dry) amount of thedispersing agent used may generally be in the range of from 0.01% to2.0% by weight, usually from 0.02% to 1.5% by weight, based on the dryweight of carbonate present.

In step (d) the thermal evaporation is preferably effected by firstraising the temperature of the suspension formed in step (b) to withinthe range of from 60° C. to the boiling point of the suspension,preferably in the range of from 80° C. to 90° C., by a heating meanssuch as a heat exchanger, and then exposing the heated suspension to areduced pressure, preferably to a vacuum of at least 600 mm of mercurybelow atmospheric pressure (−0.800 bar), more preferably at least 675 mmof mercury (−0.900 bar), and most preferably at least 700 mm of mercurybelow atmospheric pressure (−0.933 bar). Conveniently the suspensionformed in step (c) is passed through one side of a non-contact heatexchanger through the other side of which is passed a hot fluid,preferably at a temperature in the range of from 60° C. to 100° C. Thehot fluid is preferably steam, and is, where possible, waste steam whichhas been generated in another part of the plant at which the process ofthe invention is being carried out.

As water is removed from the suspension by thermal evaporation in step(d), the suspension becomes more viscous through the formation of a gelstructure in the suspension, and it becomes necessary to subject thesuspension to the mechanical working in step (e), preferably withdispersing agent present, in order to break down the gel structure andto restore the desired degree of fluidity to the suspension whereby thesuspension may be further dewatered, e.g. by a re-circulating a pumpedflow of the suspension through the means providing the evaporativedewatering. Optionally, the working may be accompanied by furtherdispersing agent addition.

The amount of energy dissipated in the suspension during the mechanicalworking step (and in each subsequent mechanical working step optionallyapplied) is at least 1 kW.hr, and is preferably in the range of fromabout 10 kW.hr to about 100 kW.hr, per tonne, on a dry weight basis, ofcarbonate in the suspension. Each working step may be applied by passingthe suspension through one or more high shear agitation devices capableof dissipating the required rate of energy. The high shear agitationdevice(s) may be, for example, a mixing vessel equipped with a rotatinginternal impeller, and/or it may be a high power centrifugal pump whichis capable of dissipating the required amount of energy in thesuspension.

Where two or more working steps are applied to the suspension, thedifferent working steps may be carried out by the same or differentdevices.

The required amount of dewatering may be applied to the suspension instep (d) by passing the suspension through heating means and throughreduced pressure evaporative dewatering means at least twice. Thesuspension.:may be recirculated to make multiple passes through the sameheating and evaporation means The suspension may also make multiplepasses through a mixing vessel where dispersing agent is added and/orthrough a device in which the suspension is worked following evaporativedewatering.

The or each thermal evaporation treatment of the suspension ispreferably effected in a flash tank which is connected to a condenserfor water vapour, both the flash tank and the condenser being evacuatedto a vacuum of at least −0.8 bar by means of a vacuum pump. The degreeof vacuum applied to the flash tank and the condenser is convenientlycontrolled by means of a valve through which small quantities of air atatmospheric pressure can be admitted into a conduit which connects thevacuum side of the vacuum pump to the flash tank and the condenser. Thethermal evaporation system is preferably of the forced circulation typewith dewatered product being continuously drawn off at a convenientpoint in the recirculation loop. It may be advantageous, for reasons ofeconomy, to use more than one stage of flash tank and condenser in therecirculation loop.

The method of the invention may be carried out as a batch,semi-continuous or continuous process.

The present invention allows a carbonate, especially a precipitatedcalcium carbonate, to be prepared in the form of a dilute suspension andthen dewatered to give a concentrated suspension which is flowable,pumpable and transportable and is Theologically stable form at maximumsolids. The invention allows the concentrated suspension to be preparedmore effectively and in a more economical manner than the methods of theprior art aimed to maximise solids content.

Although the individual steps employed in the method of the inventionare known per se (although evaporative dewatering is not used widely onaccount of its complexity and cost) the selection and use together ofthese steps as a beneficial combination has not been contemplated in theprior art. In particular, use of step (b) to apply a first stage ofdewatering to concentrate the suspension to an intermediate solids levelfollowed by a combination of evaporative dewatering and subsequentmechanical working following dispersing agent addition to complete thedewatering process, allows a higher maximum solids level to be attainedin a manner more cost effectively than in prior art processes.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the present invention will now be described by way ofexample with reference to the accompanying drawing in which:

FIG. 1 shows a diagrammatic flow sheet of an arrangement of apparatus ina plant for carrying out a process embodying the invention as applied tothe concentration of a precipitated calcium carbonate suspension.

As seen in FIG. 1, a dilute suspension of a precipitated calciumcarbonate is supplied to a plate filter press 1 through a conduit 2.When the flow of filtrate has substantially stopped, the cakes formed inthe filter press 1 are dropped into a mixing tank 3 provided withrotating stirring means 4. An appropriate amount of a concentratedsolution of a dispersing agent for the calcium carbonate is suppliedthrough a conduit 5. As a result of mixing the filter cakes with thedispersing agent, a concentrated suspension of calcium carbonate havinga solids concentration in the range of from about 50% to about 601 byweight of calcium carbonate on a dry weight basis is formed. Thisdispersed suspension is pumped through conduits 6 and 7 by means of apump 8 to one side of a plate type heat exchanger 9. The pump 8 isconveniently of the single-rotor screw pump type, for example of theMoyno or “MONO”™ type. This type of pump comprises a metal rotor whichrotates within and coaxially with an elastomeric stator, the rotor andstator being of such design that the fluid passing through the spacebetween them is compelled to follow a substantially helical path,thereby providing a high shear working action on the suspension. To theother, heating fluid, side of the heat exchanger 9 steam is suppliedthrough a conduit 10. The steam may be waste steam from another processon the same site, or it may, alternatively, be raised specially for thispurpose in a boiler. The flow of steam is controlled by means of a valve11, and the pressure of the steam is monitored by means of a pressuregauge 12. The heated concentrated suspension passes through a conduit 13to a flash tank 14 which is in communication through a side arm 15 witha condenser 16. The pressure in the flash tank 14 and in the condenser15 is reduced to at least −0.8 bar relative to atmospheric pressure bymeans of a vacuum pump 17, which communicates with the condenser 16through a conduit 18. The degree of vacuum which is exerted in the flashtank and in the condenser is controlled by means of a valve 19 throughwhich a small quantity of air at atmospheric pressure can be admittedinto the conduit 18. In the interior of the condenser 16 there isprovided a heat exchange coil to which cold water is supplied by aconduit 20, the water being withdrawn through a conduit 21. Condensatecollecting in the bottom of the condenser 16 is withdrawn through aconduit 22 by means of a pump 23 and is discharged to a drain through aconduit 24. A mixture of steam and condensed water is withdrawn from theheating fluid side of the heat exchanger 9 by means of the pump 23through a conduit 25 and a condensate separator 26. Uncondensed steam ispassed through a conduit 27 to the condenser 16, while separated wateris discharged by means of the pump 23 to the drain. Concentrated calciumcarbonate suspension which collects in the bottom of the flash tank 14is returned through conduits 28 and 29 to a mixing tank 31, which isprovided with an internal rotating impeller 32, by means of a powerfulcentrifugal pump 30, in which the suspension is subjected to high shearworking action.

A further amount of energy may also be dissipated in the suspension bymeans of the rotating impeller 32 in the mixing tank 31. Concentratedsuspension is continuously withdrawn from the bottom of mixing tank 31through a conduit 33, which communicates with conduit 6, by means ofpump 8, and is recirculated through the heat exchanger 9 to the flashtank 14. Meanwhile a portion of the recirculating stream of suspensionmay be withdrawn through a valve 34 and conduit 35 as dewatered product.

A dilute suspension of aragonitic precipitated calcium carbonate wasprepared and then treated by a process using apparatus as illustrated inFIG. 1. The calcium carbonate was prepared by the known method ofcarbonating a dilute aqueous suspension of calcium hydroxide with carbondioxide gas. The suspension contained 19.9% by weight of dry calciumcarbonate and its pH was 8.3. The calcium carbonate prepared had aparticle size distribution such that 75% by weight consisted ofparticles having an equivalent spherical diameter smaller than 0.5 μm.The dilute suspension was first dewatered in a plate filter press at apressure of 60-80 psig (414-552 kPa) to form cakes having a solidscontent of 50-52% by weight of calcium carbonate. These cakes were thendischarged into a mixing tank provided with an internal rotatingimpeller where they were mixed with a concentrated aqueous solution of asodium polyacrylate dispersing agent to form a fluid suspension. Theeffective amount of the dispersing agent used was 0.5% by weight ofactive sodium polyacrylate, based on the dry weight of the calciumcarbonate. The suspension thus formed was passed through one side of aplate heat exchanger to the other side of which was fed steam at atemperature in the range of from 82° C. to 90° C. The heated suspensionwas then passed through a vacuum evaporator in which the vacuum wasmaintained in the range of from −0.935 to −0.95 bar. Finally theconcentrated suspension was returned to the mixing tank to be furtherrecirculated. During the recirculation, further additions of sodiumpolyacrylate dispersing agent were made as were found necessary tomaintain acceptable fluidity of the suspension. On completion of thedewatering process, the total effective amount of sodium polyacrylatedispersing agent present, including the amount initially added, wasabout 1.1% by weight, based on the dry weight of the calcium carbonate.

When the solids concentration of the recirculating suspension hadincreased to about 62% by weight of dry calcium carbonate, it was foundthat the suspension had become unacceptably Theologically dilatant, andthe suspension was drawn off into a liquid working tank equipped with ahigh shear internal impeller, where high shear-mixing was performed fora time such that 23 kW.hr of energy per dry tonne of calcium carbonatewas dissipated in the suspension. The suspension, which had again beenrendered sufficiently fluid for further treatment, was returned to therecirculation system comprising the mixing tank, heat exchanger andvacuum evaporator, and was recirculated until the solids concentrationhad increased to 71% by weight. The suspension was then drawn off againinto the liquid working tank where it was subjected to high shear mixingfor a time such that 24 kW.hr of energy per dry tonne of calciumcarbonate was dissipated in the suspension. The total amount of energydissipated in the suspension was thus 47 kW.hr per tonne of calciumcarbonate on a dry weight basis.

At the completion of the second liquid working step, the suspension wasfound to be sufficiently fluid to allow it to be pumped through pipes orhoses, and was in a suitable form for transport in slurry form, e.g. ina transport tank. It was also found that the resulting suspension couldbe stored for 7 days without significant increase in viscosity.

What is claimed is:
 1. A method of producing a rheologically stable concentrated aqueous suspension of a particulate alkaline earth metal carbonate which method comprises the following steps: (a) preparing a dilute aqueous suspension of the carbonate having a solids content of not more than 40% by weight; (b) dewatering the dilute aqueous suspension to form a carbonate suspension having a solids content in the range of from 45% to 65% by weight; (c) further dewatering the fluid suspension formed in step (b) by thermal evaporation under reduced pressure to raise the solids content of the suspension by a further differential amount of at least 5% by weight; and (d) after at least some of the dewatering in step (b) treating the suspension by a mechanical working process in which at least 1 kW.hr per tonne of carbonate on a dry weight bass is dissipated to refluidise the suspension.
 2. A method according to claim 1, wherein at least two working steps each preceded by evaporative dewatering under reduced pressure are applied to the suspension following step (b).
 3. A method according to claim 1 or claim 2, wherein at least two dispersing agent additions are made to the suspension following step (b) at least one such addition being after at least some evaporative dewatering under reduced pressure.
 4. A method according to claim 1, wherein working and/or dispersing agent addition is applied when the suspension has reached its maximum solids concentration following step (c).
 5. A method according to claim 1, wherein the energy dissipated during the working is at least 10 kW.hr per dry tonne of carbonate.
 6. A method according to claim 1, wherein the suspension has a solids content of not more than 25% by weight prior to step (b), a solids content of from 50% to 65% following step (b) and a solids content of at least 68% by weight following step (c).
 7. A method according to claim 1, wherein in step (b) the dewatering is effected by means of a plate filter press, a vacuum filter, a membrane cross-flow filter or a centrifuge.
 8. A method according to claim 1, wherein dispersing agent is added in at least one step prior to or during step (c) in a total effective amount of from 0.1% to 2% by weight based on the dry weight of the carbonate present and the dispersing agent is introduced in the form of an aqueous solution.
 9. A method according to claim 1, wherein in step (c) the suspension is passed through a heating device wherein the temperature thereof is raised to at least 60° C., after which the suspension is passed into a vessel in which it is exposed to a reduced pressure.
 10. A method according to claim 9, wherein the temperature of the suspension is raised to at least 80° C. by passage through the heating device.
 11. A method according to claim 9, wherein the pressure is reduced in the said vessel below atmospheric pressure by an amount of at least 600 millimetres of mercury.
 12. A method according to claim 9, wherein multiple passes of the suspension by recirculation are made through the heating device and the said vessel.
 13. A method as claimed in claim 1, wherein the carbonate comprises a precipitated calcium carbonate.
 14. A method as claimed in claim 1, further comprising the step of mixing with the dewatered suspension formed in step (b) a dispersing agent for the carbonate to form a fluid suspension. 